188 INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



to excess potassium of the A and B fibers in the bullfrog sciatic nerve is 

 maintained even at low temperatures. This statement is supported by the 

 evidence in the electrograms of figure 7. Records 1 to 11 are the A and B 

 waves obtained from a desheathed nerve at 1.1 °C. Record 1 is the control 

 obtained before adding the test solution to the nerve segment. A solution 

 with KCl at 8.5 times the normal concentration was then added with the 

 results shown in records 2-7. Over this period of time the A activity 

 ceased almost entirely while the B potential was only slightly modified. 

 Recovery took place rapidly and completely (records 8-11) following the 

 addition of Ringer's solution to the nerve segment. The responses of the 

 same nerve to the same test solution but at normal temperature (22.7°C) 

 are arranged alongside in records 12-21. Only the time scale over which the 

 action took place in the two experiments was different. The selective 

 sensitivity of the A and B groups was not essentially altered by the low 

 temperature. This similarity in the case of the responses to potassium is 

 interesting because it is in sharj) contrast to the behavior of the A and B 

 groups to sodium. 



RESPONSES TO SODIUM — A FIBERS 



It is a commonly known fact that solutions with a deficiency of sodium 

 ions produce a conduction block in nerve fibers (12, 17). The manner in 

 which this low sodium block of the A fibers was influenced by cold is il- 

 lustrated in figure 8. The four steps of this experiment were performed 

 consecutively as numbered, using the normal temperature (22°C) for the 

 initial step. For each of these steps a solution was added at zero time which 

 had, in place of the NaCl of Ringer's fluid, an osmotically equivalent 

 amount of TMA. The other constituents were present as in Ringer's solu- 

 tion. Conduction block was rapid in all four steps of this experiment and 

 though there appears to be a slight slowing of the rate of block by low 

 temperature, it is doubtful whether this represents a significant effect. 

 In 13 experiments of this type (table 2) the time to 50% block at normal 

 temperatures averaged 7.7 minutes with a range of 4.7-19.4 minutes. At 

 the low temperature the average was 11.1 minutes and the range was 6.6- 

 16.4 minutes. The difference was not at all striking and it is questionable 

 whether any significance can be attached to the difference of the means. 

 The more interesting and striking phenomenon was the manner in which 

 recovery from the low sodium block w^as slowed or even completely in- 

 hibited by cold. These recovery effects were most conveniently studied, 

 as indicated previously (5) , not by adding Ringer's solution, but by means 

 of a solution with a limiting concentration of NaCl. At normal tempera- 

 tures a useful concentration for this purpose was found to be NaCl at 

 11 niM. With such a concentration of sodium ions it was previously found 



